The present invention is related to circuits generally, and more specifically to a method for tuning a transconductance amplifier.
Transconductance filters are used in a variety of electronic circuits that perform filtering functions. Typically, these filters incorporate a transconductance amplifier that receives a voltage input signal and outputs a controlled current signal, where the controlled current signal equals the product of the transconductance gain gm of the amplifier and the voltage input signal applied to the amplifier.
A variety of methods have been used in the prior art to tune transconductance filter circuits. U.S. Pat. No. 6,172,569 describes use of a potentiometer to adjust the amplitude of the analog control signal applied to a transconductance amplifier, as well as use of a digital-to-analog converter (DAC) to convert a digital control signal to an analog signal prior to applying it to the transconductance amplifier, thus allowing digital control of the transconductance filter characteristics. By varying the control signal applied to the transconductance amplifier, the maximum gain of the transconductance filter can be adjusted, which, shifts the unity gain frequency along the frequency axis.
Mehr et al., xe2x80x9cA CMOS Continuous-Time Gm-C filter for PRML Read Channel Applications at 150 Mb/s and Beyond,xe2x80x9d IEEE Journal of Solid-State Circuits, Vol. 32, No. 4, Apr. 1997, pp 499-513 describes a tuning circuit that requires a relatively accurate reference clock period. In such a circuit, some of the clock signals can become coupled into the actual filter through the control voltage line. This can introduce undesirable signal components in the filter, producing unwanted side bands and frequency modulation of the signal that""s being processed.
An improved method and circuit for adjusting a transconductance amplifier is desired.
One aspect of the invention is a method for tuning a transconductance amplifier. A first current is output from an output of a first transconductance amplifier. A second current is output from an output of a second transconductance amplifier that is coupled to the output of the first transconductance amplifier. The transconductance of the second transconductance amplifier is adjusted until the second current is equal in magnitude to and opposite in polarity from the first current.
Another aspect of the invention is a circuit for tuning a transconductance amplifier, comprising a first transconductance amplifier outputting a first current from an output thereof, a second transconductance amplifier outputting a second current from an output that is coupled to the output of the first transconductance amplifier, and a feedback loop. The feedback loop provides a control signal that varies with a difference between the first current and the second current and is used to adjust the transconductance of the second transconductance amplifier.